The invention relates to a process for producing a pulp from cellulose-containing material, the pulp itself and its application.
By pulp is meant a cellulose mash that is used in the production of chemical pulp, which, in turn, is used to make paper or cardboard. The cellulose-producing industry today is finding it increasingly difficult to meet the environmental standards and requirements imposed on it. In addition, it is no longer permitted in Germany, by virtue of the environmental regulations of 1990, to use conventional processes for recovering cellulose, such as the sulphite process. In Germany today the sulphite process is the only process wherein sulphur-containing digestion agents are used to dissolve the lignin, which serves as a binder, out of the cellulose-containing material.
One result of efforts to develop more environmentally-friendly processes for recovering cellulose is the so-called Acetosolv process, in which the digestion liquid contains at least 50 percent by weight of acetic acid, to which is added a very small quantity of hydrochloric acid. Following digestion, the recovered pulp is washed with caustic soda and, if required, with organic solvents, in order to more or less completely remove the lignin.
The disadvantages of this process are the relatively high consumption of acetic acid, the use of caustic soda, and the organic solvents that may be needed to wash the pulp.
In accordance with the stringent requirements of the paper industry, the extracted pulp is regularly bleached in a subsequent step, in order to keep the Kappa number at the most below 25. Even the Acetosolv process includes downstream bleaching with peroxide, with respect to which it should be pointed out that high use is not advantageous.
EP 0 325 891 A1 discloses an improvement to this Acetosolv process. The improvement comprises in essence that the pulp be washed following the digestion step, not with caustic soda, but with a C.sub.1-3 -carboxylic acid, or with a mixture of such acids, whereupon the subsequent bleaching step is carried out in the existing acid medium, to which hydrogen peroxide or ozone is added. A carboxylic acid such as, for example, butyl acetate, can be used as the solvent. This patent also discloses that the C.sub.1-3 -carboxylic acid can later be re-used as a digestion liquid.
Disclosed in EP 0 250 422 B1 is a process commonly known as the Milox process, in which bleached pulp is produced from cellulose-containing material. In this process, digestion is accomplished in a peroxyformyl, peroxyaceto, peroxypropional or peroxybutyric acid medium, wherein recovery of peroxic acid is effected in that the acid used is mixed with a relatively large quantity of hydrogen peroxide, whereupon the digestion process continues. Following digestion, the pulp is bleached in an alkali solution to which hydrogen peroxide has been added. The disadvantage of this process, however, is the use of an alkali solution such as caustic soda, as well as the high proportion of hydrogen peroxide required.
Using this prior art as the basis, the present invention aims to develop a process for producing a pulp from cellulose-containing material that is environmentally-friendly, highly economical and efficient.